Troubleshooting an amp? Read this before you post your ques

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Troubleshooting an amp? Read this before you post your ques

Post by øøøøøøø » Sun Aug 31, 2008 8:49 pm

So on this forum you see a lot of "help me troubleshoot my amp" type of posts.   Since I seem to be among the biggest dorks on this section of the forum I end up answering a lot of those posts and find myself saying some of the same things over and over.

Maybe it would be useful to make this a sticky, if the mods deem it worthy.  :)

First and foremost, a little background information is in order.  Here are some things to keep in mind if your amp has suddenly crapped out:

1) I'd estimate roughly 90% of tube amp problems are tube related.  There's a reason those things are in sockets and can be removed and replaced.  They fail more than any other component.  Tube failures can manifest themselves in almost any way imaginable, from blowing fuses to low output, bad sound, undesired distortion, crackling or hum.  They are not light bulbs.  Just because a tube "lights up" does not mean it is good.
2) If your amp is 20 years old or more and has never had a cap job, then I'd lay odds that most of the non-tube-related problems that it could have will be electrolytic capacitor related.  You should get the amp serviced.
3) There are many things you can do to narrow down the problem or even repair (or at least diagnose) it yourself before posting here or paying a tech.  I'll outline some of those below
4) Many problems are easy to fix and user-servicable, BUT if you have to open up your amp, remember that there can be lethal amounts of stored electricity in the amp even when not plugged in.  Most things like replacing tubes, etc. are perfectly safe.  If you have to open the chassis, though, you should be careful.

OK, so on to the troubleshooting basics!


The first common problem is an amp that is popping fuses.  The most common causes of this are a) bad tube of course ;) b) bad diode, if solid-state rectifier, c) bad filter capacitor--of course d) short in the wiring somewhere, e) faulty power transformer.

Note that nowhere in there is "bad fuse."  If a fuse blows, DO NOT just stick a new fuse in and try again.  You'll be wasting fuses and putting the amp under further strain.  A fuse is there to shut the amp off to protect it when there's a real problem.

If your amp is blowing fuses, here's what you do!

STEP ONE: make sure you have 3 or 4 fuses of the appropriate current rating (amperes) on hand.  Voltage rating of a fuse doesn't matter, it protects against current, not voltage.  Make sure to use slo-blo fuses, if your amp calls for them (most do) or you might blow fuses even when nothing is wrong. 

STEP TWO: remove ALL TUBES from the amp and try powering on again.  Does the amp blow a fuse?  If not, proceed to step three.  If so, read on:

Step 2a: If the amp still blows fuses with no tubes installed, you should see a tech.  If your amp is tube rectified, you most likely  have either a shorted filter cap or a shorted power transformer.  If your amp is not tube rectified, it's more likely that you have a shorted rectifier diode.  The transformer is the only one that is really bad news.  It's also not terribly common, but it does happen.  Usually if the transformer goes bad, something else (like a bad tube or cap) took it out.  If you are reading this, you don't know how to replace a transformer and shouldn't be messing with it.  If you want to try and replace the rectifier diodes or filter caps, you can ask on this forum or read the DIY sticky threads!  :)

STEP THREE: If the fuse holds with no tubes in, good news!  Your problem is a tube problem.  First, if your amp is tube rectified, install the rectifier tube only.  If your amp is solid-state rectified, you may skip that step.  Does the fuse hold with the rectifier tube in place?  Good!  You have ruled out the rectifier.  Proceed to step 3a.

Step 3a: Either replace the power tubes or try to re-install them.  Most likely the fuse will blow if you re-install them.  If the rectifier tube is good, the power tubes are probably what's causing the fuse to blow.  If you re-insert the old power tubes and the fuse doesn't blow, insert the preamp tubes one at a time until one blows the fuse (or they are all in).  If the fuse holds, either you had a fluke or have an intermittent problem.  If it blows another fuse, I'd replace the power and rectifier (if applicable) tubes and see if the problem goes away.  If not, tech time.

Other, rarer causes of popping fuses could be carbon trails on power tube sockets, bias supply cap or diode shorted or open, in rare cases an ultrasonic oscillation, etc.  In any case, that stuff is for a tech to track down unless you're feeling ambitious.  But if you're reading this, you're probably under-qualified, unfortunately.


The second one people always ask about is extraneous noises, mainly hum and/or hiss

This could be a million things, but guess what?  It's probably a tube.  If it's not a tube, it's probably a cap.  See how easy tube amps are?  :)  You can track it down and narrow it down somewhat. 

STEP ONE: Make sure it's the amp.  Unplug all cables, pedals, guitars etc. from the input of the amp and have just the amp on.  If it doesn't make noise in this state, it's not the amp's problem.  Try a different outlet or room, too, unless the amp worked normally in that location before.

STEP TWO: see what, if any, controls on the amp affect the noise.  Does it get louder or softer as the volume control is manipulated?  Do the tone controls affect it?  What about the reverb control?  Master volume (if applicable)?  Is the noise on both /all channels in a multi-channel amp?

If the volume or tone controls affect it, the culprit is near the very beginning of the amp.  If the noise disappears entirely when the volume is at zero, it's something in the first gain stage, in most amps.  Try replacing that tube.   If the reverb affects it, look at tubes, cables, tank, etc. in the reverb circuit.  If you have a master volume, and the MV doesn't affect the noise, then it's probably something in the power amp or maybe phase inverter, depending on the design of the MV circuit.

STEP THREE: Once you have isolated the area of the amp where the problem might be (or decided that the above test was inconclusive), start pulling tubes and see when the noise stops!  If you're still not sure about the test above, start with the first preamp tube(s) which are usually farthest from the power tubes.  Then proceed "down the line" in order working your way towards the power tubes.  When you get to the point where the noise stops if you pull that tube, try substituting that tube. Or at least, try re-inserting it--maybe it was just loose or the socket was just dirty.  Noise back?  Try replacing the tube with another.  No spare?  Swap it with another from somewhere else in the amp and see if the problem "moves."  You can figure out a lot this way.  In 90% of cases, the problem is licked at this step.

STEP FOUR: If you have determined that the noise is not tube related--you have swapped all the tubes at least temporarily and re-seated them in their sockets without any change in the problem-- we can talk about some of the other things that can cause noise issues.

If your amp has not had filter caps in the last 15 years and it has developed a loud hum, then you need filter caps.  It's about a 95% chance that's what's causing your hum, if it's not a tube.  If it's been that long, you need them anyway.  Do it.

If your amp has fresh caps and the tubes are good, but you are still getting noise problems--particularly if the nosies are sporadic/intermittent--try wiggling the tubes in their sockets to see if you can induce the noise.  If you can, your tube sockets need to be cleaned and re-tensioned (the contacts bent to hold the tube pins tighter, usually with a dental pick AFTER the caps have been properly discharged for safety!!)

If you have a non-tube/non-socket-related hiss problem and it's an older amp, it's likely that the plate load resistors have absorbed moisture.  Tech time, or ask here if you want to know how to replace them yourself-- not too hard on most vintage amps.

-----------------

That is the main, most common stuff people ask for help with.  That's the stuff that I've typed out several times in several posts with people asking the same sort of stuff. 

For something not listed here, I'd suggest you check out the tube amp debugging page.  It's arranged in a handy flow-chart style.  If you see something on there you don't understand, ask here.

Alright, cool!
:)  :)  :)
Last edited by øøøøøøø on Wed Sep 29, 2010 10:29 am, edited 3 times in total.

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Re: Troubleshooting an amp? Read this before you post your question!

Post by mezcalhead » Sun Aug 31, 2008 9:40 pm

*rips off length of tape*

Is now a sticky, thanks Brad!
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Re: Troubleshooting an amp? Read this before you post your question!

Post by dug » Fri Sep 19, 2008 12:46 pm

Do you know anything about solid state amps?

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Re: Troubleshooting an amp? Read this before you post your question!

Post by øøøøøøø » Fri Sep 19, 2008 5:14 pm

dug wrote: Do you know anything about solid state amps?
not much!  :)

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Re: Troubleshooting an amp? Read this before you post your question!

Post by Maggieo » Fri Sep 19, 2008 7:35 pm

Dan Torres once told me that tube amps are something you repair and solid state amps are something you replace.
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Re: Troubleshooting an amp? Read this before you post your question!

Post by øøøøøøø » Fri Sep 19, 2008 7:42 pm

that's sorta true, and it goes for more than just amps.

Tube television sets, tube radios, tube hi fi amplifiers, anything with tubes in it is generally more repair-friendly.

Solid-state devices are harder to troubleshoot, and there are more esoteric and obsolete ICs and transistors, whereas most tube audio gear is built around the same two dozen or so tube types (there are exceptions).  Every year they come out with "new and improved" solid state shiv but tube technology has "settled" and is much more stable.

Also, modern solid-state stuff is usually rife with subminiature components and surface-mount devices (SMDs).  These are hard to repair/replace without specialized, expensive equipment.  Cost of replacement is usually cheaper than cost of repair.

Note that after all the tube TVs and radios had been gone from fashion for a couple of decades, you didn't see any new "TV and Radio repair" places anymore.  You certainly don't see new ones opening up.  All the old ones still remaining are steadily closing. 

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Re: Troubleshooting an amp? Read this before you post your question!

Post by SonOfJazzmasterE » Fri Sep 19, 2008 8:18 pm

Remember when you would go to the supermarket and you could just USE a tube tester>>??? That was cool!
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Re: Troubleshooting an amp? Read this before you post your question!

Post by mezcalhead » Sat Sep 20, 2008 9:26 am

Maggieo wrote: Dan Torres once told me that tube amps are something you repair and solid state amps are something you replace.
Probably nearly always true, as Brad says .. but not always!

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Re: Troubleshooting an amp? Read this before you post your question!

Post by Maggieo » Sat Sep 20, 2008 9:27 am

mezcalhead wrote:
Maggieo wrote: Dan Torres once told me that tube amps are something you repair and solid state amps are something you replace.
Probably nearly always true, as Brad says .. but not always!

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Re: Troubleshooting an amp? Read this before you post your question!

Post by tribi9 » Sat Sep 20, 2008 6:31 pm

SonOfJazzmasterE wrote: Remember when you would go to the supermarket and you could just USE a tube tester>>??? That was cool!
That must have been way before my time...  ;)
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Re: Troubleshooting an amp? Read this before you post your question!

Post by RumorsOFsurF » Wed Oct 08, 2008 8:02 pm

SonOfJazzmasterE wrote: Remember when you would go to the supermarket and you could just USE a tube tester>>??? That was cool!
Hell yes!  I'm only 25, and I remember Radio Shack had one. You could bring your tubes in to see if they were any good. 
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Re: Troubleshooting an amp? Read this before you post your question!

Post by øøøøøøø » Wed Oct 08, 2008 11:11 pm

RumorsOFsurF wrote:
SonOfJazzmasterE wrote: Remember when you would go to the supermarket and you could just USE a tube tester>>??? That was cool!
Hell yes!  I'm only 25, and I remember Radio Shack had one. You could bring your tubes in to see if they were any good. 
I just turned 28 a couple weeks ago and I have vague memories of a tube tester at the drug store around the corner from my house as a kid.  I remember being about 5 years old wondering "what's that thing with all the checkers on it?" because it had red and black phenolic tube sockets like so:

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The funny thing is, those tube testers were barely testing anything.  They would register "good" or "bad."  They tested for emissions and shorts only.  You can't match tubes or anything on an old tester like that.

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Re: Troubleshooting an amp? Read this before you post your question!

Post by SonOfJazzmasterE » Sat Oct 11, 2008 8:10 pm

I remember see those... I was really young myself... but I always thought they waz COOL!
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Re: Troubleshooting an amp? Read this before you post your question!

Post by Jay » Sun Oct 12, 2008 9:18 am

The local electronics shop I use for the occasional need it quick part has a giant tube tester in the store that you can use still.  Pretty cool...  I also used to have an old Elenco (?) unit I bought at a Goodwill but I don't know what happened to it.  :(  Of course not being able to match tubes makes them sort of pointless to a degree but you can sort out bad tubes with them I guess.  In any case, they're cool looking things that's for sure.

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Re: Troubleshooting an amp? Read this before you post your question!

Post by tribi9 » Sun Oct 12, 2008 9:36 am

There's a small local antique electronics store in town. They got 2 tube testers, they even offered if I wanted to buy the spare one. They both look like the smaller one tho' (First pic in Brad's post) :)
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